But while our chalk cliffs conjure up a vision of England lying deep--at least twenty or thirty fathoms deep--below a warm ocean, in which gigantic Ammonites and Belemnites and sharks ply their deadly trade, they also remind us of the last phase of the remarkable life of the earth's Middle Ages. In the latter part of the Cretaceous the land rises. The chalk ocean of Europe is gradually reduced to a series of inland seas, separated by masses and ridges of land, and finally to a series of lakes of brackish water. The masses of the Pyrenees and Alps begin to rise; though it will not be until a much later date that they reach anything like their present elevation. In America the change is even greater. A vast ridge rises along the whole western front of the continent, lifting and draining it, from Alaska to Cape Horn. It is the beginning of the Rocky Mountains and the Andes. Even during the Cretaceous period there had been rich forests of Mesozoic vegetation covering about a hundred thousand square miles in the Rocky Mountains region. Europe and America now begin to show their modern contours. It is important to notice that this great uprise of the land and the series of disturbances it entails differ from those which we summed up in the phrase Permian Revolution. The differences may help us to understand some of the changes in the living population. The chief difference is that the disturbances are more local, and not nearly simultaneous. There is a considerable emergence of land at the end of the Jurassic, then a fresh expansion of the sea, then a great rise of mountains at the end of the Cretaceous, and so on. We shall find our great mountain-masses (the Pyrenees, Alps, Himalaya, etc.) rising at intervals throughout the whole of the Tertiary Era. However, it suffices for the moment to observe that in the latter part of the Mesozoic and early part of the Tertiary there were considerable upheavals of the land in various regions, and that the Mesozoic Era closed with a very much larger proportion of dry land, and a much higher relief of the land, than there had been during the Jurassic period. The series of disturbances was, says Professor Chamberlin, "greater than any that had occurred since the close of the Palaeozoic."